Flow process for pretreatment of lignocellulosic biomass

ABSTRACT

Described are methods for pretreating lignocellulosic biomass that comprise thermally conditioning the biomass by flow processing an aqueous slurry of the biomass through an outer passage(s) of one or more heat exchange devices while circulating a heat exchange fluid through an inner passage(s) of the heat exchange device(s). Also described are methods for producing fermentation products, especially ethanol, from the pretreated biomass.

REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/US2009/048863, which is hereby incorporated by reference, whichclaims the benefit of U.S. Provisional Application No. 61/076,019 filedJun. 26, 2008, which is hereby incorporated by reference.

BACKGROUND

The present invention relates generally to the utilization oflignocellulosic biomass, and in certain embodiments to systems andprocesses useful for thermally treating aqueous slurries oflignocellulosic biomass under flow conditions to render cellulosetherein more susceptible to hydrolysis.

As further background, increasing emphasis has been placed in recentyears upon finding ways to efficiently produce fuels from renewable,non-petroleum resources. In one field of interest, fuel ethanol has beenproduced by fermentation of biomass feedstocks derived from plants.Currently, fuel ethanol is commercially produced from feedstocks ofcornstarch, sugar cane and sugar beets. These materials, however, findsignificant competing uses in the food industry, and their expanded useto make fuel ethanol is met with increased prices and disruption ofother industries. Alternative fermentation feedstocks and viabletechnologies for their utilization are thus highly sought after.

Lignocellulosic biomass feedstocks are available in large quantities andare relatively inexpensive. Such feedstocks are available in the form ofagricultural wastes such as corn stover, corn fiber, wheat straw, barleystraw, oat straw, oat hulls, canola straw, soybean stover, grasses suchas switch grass, miscanthus, cord grass, and reed canary grass, forestrywastes such as aspen wood and sawdust, and sugar processing residuessuch as bagasse and beet pulp. Cellulose from these feedstocks isconverted to sugars, which are then fermented to produce the ethanol.

A difficulty in using lignocellulosic feedstocks is that the cellulosecontent of the biomass is caught up in a structure that inhibits theaccessibility of the cellulose to agents that convert it to sugars. Forthis reason, research has focused upon methods for pretreatinglignocellulosic biomass to enhance the susceptibility of the celluloseto conversion to sugars. Such pretreatment processes are designed tobreak the lignin seal protecting the cellulose and to disrupt thecrystalline structure of the cellulose. A variety of pretreatmentmethodologies have been explored for this purpose; including physicalprocesses such as size reduction, steam explosion, liquid hot water,irradiation, cryomilling, and freeze explosion; and chemical processessuch as acid hydrolysis, buffered solvent pumping, alkali or alkali/H₂O₂delignification, solvents, ammonia; and microbial or enzymatic methods.

Despite previous efforts relating to pretreatments for lignocellulosicbiomass feedstocks and its ultimate use in the production of ethanol,needs remain for improved and alternative biomass pretreatment processesand follow-on production of ethanol. In certain of its aspects, thepresent invention is addressed to these needs.

SUMMARY

In one embodiment, provided is a method for processing lignocellulosicbiomass. The method includes providing a heat exchanger including aninner passage received within an outer passage, with the inner passagein heat exchange relationship with the outer passage. An aqueous biomassslurry including said lignocellulosic biomass is passed through theouter passage, and a heat exchange fluid is passed through the innerpassage, so as to exchange heat between the aqueous biomass slurry andthe heat exchange fluid.

In another embodiment, provided is a method for pretreatinglignocellulosic biomass to increase its susceptibility to enzymatichydrolysis. The method includes providing a particulate lignocellulosicbiomass and mixing the particulate lignocellulosic biomass with anaqueous medium to prepare an aqueous biomass slurry including thelignocellulosic biomass at a solids level of at least 50 grams perliter. A heat exchanger is provided, including an inner passage receivedwithin an outer passage, with the inner passage in heat exchangerelationship with the outer passage. The aqueous biomass slurry ispassed through the outer passage of the heat exchanger, and a heatexchange fluid is passed through the inner passage of the heatexchanger, so as to exchange heat between the aqueous biomass slurry andthe heat exchange fluid.

Additional embodiments of the invention as well as features andadvantages thereof will be apparent from the descriptions herein.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram of a heat exchange system useful forpretreatment of a lignocellulosic biomass slurry.

FIG. 2 is a schematic diagram of a lignocellulosic biomass pretreatmentsystem with a plurality of heat exchangers.

FIG. 3 provides a perspective view of a heat exchange tube arrangementof the invention with a helical groove in an outer passage.

FIG. 4 shows the heat up profile for heating a slurry of biomass in theshell of a shell and tube heat exchanger.

FIG. 5 shows the heat up profile for heating a slurry of biomass in thetube of a shell and tube heat exchanger.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to certain embodiments andspecific language will be used to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as described herein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

As disclosed above, certain aspects of the present invention relate tomethods and systems for pretreating lignocellulosic biomass in anaqueous slurry. Additional aspects of the invention relate to processesutilizing pretreated biomass materials in the production of products,including fermentation products such as ethanol.

As used herein, the term “lignocellulosic biomass”, is meant to refer toany type of biomass comprising lignin and cellulose such as, but notlimited to, non-woody plant biomass, agricultural wastes and forestryresidues and sugar-processing residues. For example, the cellulosicfeedstock can include, but is not limited to, grasses, such as switchgrass, cord grass, rye grass, miscanthus, mixed prairie grasses, or acombination thereof; sugar-processing residues such as, but not limitedto, sugar cane bagasse and sugar beet pulp; agricultural wastes such as,but not limited to, soybean stover, corn fiber from grain processing,corn stover, oat straw, rice straw, rice hulls, barley straw, corn cobs,wheat straw, canola straw, oat hulls, and corn fiber; and forestrywastes, such as, but not limited to, recycled wood pulp fiber, sawdust,hardwood, softwood, or any combination thereof. Further, thelignocellulosic biomass may comprise lignocellulosic waste or forestrywaste materials such as, but not limited to, paper sludge, newsprint,cardboard and the like. Lignocellulosic biomass may comprise one speciesof fiber or, alternatively, a lignocellulosic biomass feedstock maycomprise a mixture of fibers that originate from differentlignocellulosic materials.

Typically, the lignocellulosic material will comprise cellulose in anamount greater than about 2%, 5% or 10% and preferably greater thanabout 20% (w/w) to produce a significant amount of glucose. Thelignocellulosic material can be of higher cellulose content, for exampleat least about 30% (w/w), 35% (w/w), 40% (w/w) or more. Therefore, thelignocellulosic material may comprise from about 2% to about 90% (w/w),or from about 20% to about 80% (w/w) cellulose, or from 25% to about 70%(w/w) cellulose, or about 35% to about 70% (w/w) cellulose, or more, orany amount therebetween.

Prior to pretreatment, the lignocellulosic biomass can be mechanicallyprocessed to increase its surface area. Such mechanical processing mayinclude, for example, reducing the biomass to a particulate by grinding,milling, agitation, shredding, or other types of mechanical action.

In certain aspects, the lignocellulosic biomass will be used to create apumpable slurry in combination with a suitable liquid, preferably anaqueous medium. The aqueous medium may be water alone, but in otherembodiments can include additives to enhance the pretreatment processsuch as acids or bases to adjust or maintain the pH of the aqueousmedium. Aspects of the present invention are also applicable to systemswhich employ dilute acid pretreatment processes. Suitable acids forthese or other purposes herein include for example inorganic or organicacids, e.g. sulfuric, hydrochloric, phosphoric nitric, acetic, citric orformic acid. Suitable bases for these purposes include for examplealkali or alkaline earth metal hydroxides, e.g. sodium or potassiumhydroxide, or other hydroxide bases such as ammonium hydroxide. Incertain preferred forms, the aqueous medium will be adjusted initiallyand/or during a thermal pretreatment process by the addition of acid orbase to provide a pH that is near neutral, so as to avoid the occurrenceof any significant acid- or base-catalyzed autohydrolysis of thelignocellulosic material, for example a pH in the range of about 5 toabout 8. Additional information as to suitable conditions forpH-controlled lignocellulosic biomass pretreatments is found in U.S.Pat. No. 5,846,787, which is hereby incorporated herein by reference inits entirety. Other additives that may be present in the aqueous biomassslurry include, as illustrations, surfactants, e.g. vegetable oils suchas soybean oil, canola oil, and others, to serve as intercalatingagents.

The aqueous slurry of the lignocellulosic biomass will typically berelatively highly concentrated in solids. In certain embodiments, theaqueous slurry will be comprised at least about 10 grams per liter (g/1)of lignocellulosic biomass solids, preferably at least about 50 g/l,more preferably at least about 100 g/l, and typically in the range ofabout 100 g/l to about 300 g/l. It will be understood, however, thatother solids concentrations may be used in broader aspects of theinvention.

In aspects of the invention, the aqueous biomass slurry will besubjected to thermal heating and cooling cycles during pressurized flowthrough the inlets and outlets of a plurality of (two or more) heatexchangers. In such processing, the treatment system will be designed tofacilitate maintaining the aqueous slurry under a pressure substantiallyat or above its saturation vapor pressure, so as to maintain the waterand/or other liquid(s) of the slurry in liquid form as much as possible.However, at times, including during startup, passages within the systemmay encounter localized reductions in pressure due at least in part tothe extended nature of the passages within the heat exchangers. This inturn may result in localized vaporization of water and/or other liquidswhich may cause undesired chemical changes in the biomass slurry andundesired phenomena in the system components. To address this problem,in certain embodiments, systems of the invention include one or morefeed points at position(s) between the inlet and outlet of a heatexchanger passage, into which a pressurized liquid medium, suitably anaqueous medium, is fed into the heat exchanger passage carrying theaqueous biomass slurry. This feed of a pressurized liquid can serve tolocally regulate the pressure in the passage and reduce the incidence oflocalized pressure drops in the system. Alternatively, a feed ofpressurized gas, e.g. nitrogen or carbon dioxide gas, can be employed.

A wide variety of heat exchangers may be used in systems and methods ofthe present invention. These include, for example, simple tube-in-tubeheat exchangers and core-and-shell heat exchangers. In accordance withembodiments of the invention, the aqueous biomass slurry flow will beprovided in the outer passage or passages of the heat exchanger, forexample, the annular outer passage in a simple tube-in-tube exchanger,or the shell of a core-and-shell heat exchanger, while the heat exchangefluid flow will be provided within the inner passage or passages. Theflow of materials in the inner and outer passages can be co-current orcounter-current in a given heat exchanger.

In systems including multiple heat exchangers, the aqueous biomassslurry flow will be provided in the outer passage of at least one heatexchanger of the system, in certain embodiments in two or more heatexchangers of the system, and in certain preferred embodiments in allheat exchangers of the system. Thus, in some embodiments having multipleheat exchangers, one or more heat exchangers of the system may includethe aqueous biomass slurry flow in the inner passage(s) while the heatexchange fluid flow can occur in the outer passage(s). Combinations offlow patterns in multiple heat exchange devices of the system, e.g.connected in series, may be used. Further, in certain preferred aspects,in at least one heat exchanger of the system, and optionally multipleheat exchangers, to effect heat recovery, a first amount of aqueousbiomass slurry at a relatively lower temperature is passed through theinner passage(s) while a second amount of aqueous biomass slurry at arelatively higher temperature is passed through the outer passage(s), soas to exchange heat between the first and second amounts of aqueousbiomass slurry.

In all of the above-mentioned embodiments, the flow in the passages ofthe heat exchanger of the aqueous biomass slurry and the heat exchangerfluid may be reversed. The heat exchanger fluid can be fed into theinner passage of a heat exchanger while the aqueous biomass slurry ispassed through the outer passage of the heat exchanger. In systemsincluding multiple heat exchangers, the flow of the aqueous biomass maybe through both the inner or outer passages of different exchangers inthe system of heat exchangers, with the heat exchanger fluid passingthrough the passage not used by the aqueous biomass.

With reference now to FIG. 1, shown is one embodiment of a heat exchangesystem that is useful for pretreating a lignocellulosic biomass slurry.Heat exchange system 11 includes a core-and-shell heat exchanger 12having a core portion 13 including one or more fluid passages and ashell portion 14 including one or more fluid passages and encompassingthe core portion 13. Heat exchanger 12 includes a core inlet end 15including one or more openings communicating with the passage orpassages of core 13 and a core outlet end 16 including one or moreoutlet openings communicating with the passage or passages of core 13.The shell portion 14 of heat exchanger 12 includes a shell inlet end 17having one or more openings communicating with the one or more passagesin shell portion 14 and a shell outlet end 18 having one or moreopenings communicating with the one or more passages in shell portion14. The volume of the shell portion of the heat exchanger is in generalgreater than the volume of the tube portion of the heat exchanger. Thus,the volume ratio of the shell portion of the heat exchanger to the coreportion of the heat exchanger is at least about 3:1, or at least about5:1, typically in the range of about 5:1 to about 50:1 and preferably inthe range of about 10:1 to about 30:1.

In accordance with aspects of the present invention, heat exchanger 12also includes one or more openings, and preferably a plurality ofopenings 19, positioned between the shell inlet end 17 the shell outletend 18. A source of pressurized liquid, such as a pressurized aqueousmedium, is fluidly coupled to the one or more openings 19. Opening(s) 19are preferably relatively small, so as to minimize dilution of theaqueous slurry by the added pressurized liquid. Preferably, within agiven heat exchanger, opening(s) 19 will be sized and numberedsufficiently small so as to result in a dilution of the aqueous slurryin that heat exchanger by no more than about 5% (w/w), more preferablyno more than about 2% (w/w). For these purposes, in certain embodiments,the opening(s) 19 will have a cross-sectional area (considered together)of less than about 20% of the cross-sectional area of the shell (orouter) passage portion 14 of the heat exchanger, more preferably lessthan about 10%.

The source of pressurized liquid or gas can include a feed line or lines20 connected via a valve 21 to a pressurized tank 22. Pressurized tank22 can include an incompressible liquid 23 such as an aqueous medium,and a pressurized gaseous atmosphere 24 exerting pressure upon theliquid 23. Pressurized atmosphere 24 can, for example, comprise watervapor, air, nitrogen, or any suitable gas or combination of gases forproviding a gaseous environment that exerts pressure on liquid 23. Apressure relief valve 25 can be provided on tank 22, as well as a bleedvalve 26 for bleeding the system as necessary.

Heat exchange system 11 also includes a source 27 of an aqueous slurryof a lignocellulosic biomass material, and a pump 28 for pumping theaqueous slurry under pressure through the shell side of heat exchanger12. System 11 includes a source 29 of a heat exchange fluid coupled tothe core side of heat exchanger 12. Source 29 can provide the heatexchange fluid in gaseous or liquid form and at a temperature that ishigher or lower than that of the aqueous biomass slurry. In certainembodiments disclosed herein, the source 29 provides the heat exchangefluid in the form of a liquid aqueous medium or steam. The heat exchangefluid from source 29 is circulated through the core portion 13 from coreinlet end 15 to the core outlet end 16.

In use, the aqueous biomass slurry from source 27 can be circulatedthrough the shell portion 14 of heat exchanger 12 at a pressuresubstantially at or exceeding its saturation vapor pressure in order tofacilitate maintaining the water and/or other liquid of the slurry inliquid form. However, as the aqueous biomass slurry passes through theshell portion 14 of heat exchanger 12, localized pressure drops maydevelop which may lead to vaporization or flashing of water and/or otherliquid from the aqueous slurry. This in turn interrupts the desiredliquid-form modification of the lignocellulosic biomass and decreasesthe overall efficiency of the pretreatment process, and also may tend tocause “water hammer” or “bump” in the system which exerts unnecessarystresses on system components. To ameliorate the occurrence of localizedpressure drops, and incompressible liquid medium such as an aqueousliquid medium, suitably water alone, is fed under pressure from tank 22through the openings 19 at intermediate locations within heat exchanger12. This intermediate feed will be conducted at a pressure sufficient toreduce the incidence of localized pressure drops, typically at apressure at or above the initial feed pressure of the aqueous biomassslurry into the shell inlet end 17 of heat exchanger 12. Such localizedpressure regulation with an incompressible liquid may be used at anydesired time during processing of the aqueous biomass slurry throughheat exchanger 12, and may be particularly beneficial when used duringstart-up operations.

It will be understood that the source of pressurized liquid forintermediate feed to heat exchanger 12 can vary from that shown in FIG.1 in other embodiments. For example, pressurized liquid can be providedby a pump or any other suitable mechanism. Further, in accordance withadditional aspects of the invention, the pressured liquid forintermediate feed to the heat exchanger can contain additives formodifying the aqueous biomass slurry as it passes through the heatexchanger system. Such additives can include, for example, any of thosediscussed herein, including acid or base for adjusting pH, surfactants,e.g. vegetable oils such as soybean oil, canola oil, and others, toserve as intercalating agents. In certain embodiments, such additivescan be added to the pressurized liquid feed at selected times undercontrol of appropriate valving and supply systems. Other embodiments,such additives can be included as a standard measure in the pressurizedliquid feed. In these embodiments, additional control and variation ofthe pretreatment conditions for the lignocellulosic biomass can beprovided during the process.

With reference now to FIG. 2, shown is a lignocellulosic biomasspretreatment system 30 that includes a plurality of heat exchangers. Insystem 30, an aqueous slurry of a lignocellulosic biomass is passedthrough the shell sides of the heat exchangers and a heat exchange fluidis passed through the core sides of the heat exchangers. In particular,system 30 includes heat exchanger 31, heat exchanger 32, heat exchanger33, heat exchanger 34, heat exchanger 35, and heat exchanger 36, havingtheir shell sides coupled in series. As examples, core and shell orsimple tube-in-tube heat exchangers may be used. In the illustratedembodiment, each heat exchanger is fluidly coupled to a source 37 ofpressurized liquid through feed lines 38 that open into a correspondingplurality of openings in the shell sides 40 of the heat exchangers. Theheat exchangers also include a core side 39 encompassed by the shellside 40. This source of pressurized incompressible liquid can be such asthat described in the proceeding passages and can be useful to locallyregulate pressures within the system 30.

System 30 includes a source of lignocellulosic biomass 41 and pump 42for pumping the biomass in slurry form through the system 30. Asillustrated, the biomass slurry is pumped through the shell sides 40 ofthe heat exchangers of system 30, whereas various heat exchange fluids(in some cases potentially including another amount of aqueous biomassslurry or a fraction thereof) are passed through the cores 39 of theheat exchangers. As will be discussed further below, in one module ofthe heat exchange fluid side, a source of heated, liquid-form water 43is pumped by pump 44 through the cores 39 of heat exchangers 32 and 34of system 30. In another module of the heat exchange fluid side ofsystem 30, a source of steam 45, such as a boiler, is provided tocirculate steam through the core 39 of heat exchanger 33.

Generally in system 30, as the aqueous biomass slurry is pumped throughheat exchangers 31, 32, and 33, it is subjected to increasingtemperatures. For example, the temperature ST₁ of the initial drylignocellulosic biomass can be room temperature, for example about 20°C. to about 25° C. After being combined with heated liquids such as aheated aqueous medium, the temperature ST₂ of the aqueous biomass slurrycan be about 50° C. to about 90° C. The temperature ST₃ of the aqueousbiomass slurry after exiting the first heat exchanger 31 will be higherthan ST₂, typically in the range of about 70° C. to about 120° C. Thetemperature ST₄ of the aqueous slurry after exiting heat exchanger 32can typically be about 120° C. to about 170° C. The temperature ST₅ ofthe aqueous biomass slurry after exiting heat exchanger 33 can typicallybe in the range of about 150° C. to about 220° C. From this point, theaqueous biomass slurry is passed through a series of heat exchangerswhich decrease its temperature. Thus, the temperature ST₆ of the aqueousslurry as it exits heat exchanger 34 can be in the range of about 100°C. to about 150° C., the temperature ST₇ of the aqueous slurry afterexiting heat exchanger 35 can be about 50° C. to about 100° C., and thetemperature ST₈ after exiting heat exchanger 36 can typically be about30° C. to about 70° C.

On the heat exchange fluid side of the system, the temperature XT₁ ofthe heat exchange fluid exiting heat exchanger 31 can typically rangefrom about 70° C. to about 100° C., where as the temperature XT₂ of theheat exchange fluid entering heat exchanger 31 can typically be in therange of about 90° C. to about 120° C. The temperature XT₃ of the heatexchange fluid exiting heat exchanger 32 can typically be in the rangeof about 100° C. to about 150° C., whereas the temperature XT₄ of theheat exchange fluid entering heat exchanger 32 can typically be about130° C. to about 180° C. The temperature XT₅ of the heat exchange fluidexiting heat exchanger 33 can typically be in the range of about 140° C.to about 180° C., whereas the temperature XT₆ of the heat exchange fluidentering heat exchanger 33 can typically in the range of about 150° C.to about 230° C. The temperature XT₇ of the heat exchange fluid exitingheat exchanger 34 can typically be about 130° C. to about 180° C., whilethe temperature XT₈ of the heat exchange fluid entering heat exchanger34 can typically be about 100° C. to about 150° C. The temperature XT₉of the heat exchange fluid exiting heat exchanger 35 can typically be inthe range of about 90° C. to about 120° C., where as the temperatureXT₁₀ of the heat exchange fluid entering heat exchanger 35 can typicallybe about 60° C. to about 110° C. The temperature XT₁₁ of the heatexchange fluid exiting heat exchanger 36 can typically be about 60° C.to about 110° C. The temperature XT₁₂ of the heat exchange fluidentering heat exchanger 36 can typically be in the range of about 20° C.to about 70° C.

In certain processes of the invention, the lignocellulosic biomassutilized will be corn stover or corn fiber (derived from the hulls ofcorn kernels). In preferred such processes, the temperatures given inTable 1 below are applied in a system such as that described inconnection with FIG. 2:

TABLE 1 Corn Fiber Corn Stover ST XT ST XT Temperature TemperatureTemperature Temperature (±10° C.) (±10° C.) (±10° C.) (±10° C.) ST¹ 20°C. XT¹  80° C. ST¹  20° C. XT¹ 100° C. ST² 70° C. XT² 105° C. ST²  90°C. XT² 135° C. ST³ 95° C. XT³ 105° C. ST³ 125° C. XT³ 135° C. ST⁴ 140°C.  XT⁴ 150° C. ST⁴ 170° C. XT⁴ 180° C. ST⁵ 160° C.  XT⁵ 160° C. ST⁵190° C. XT⁵ 190° C. ST⁶ 115° C.  XT⁶ 170° C. ST⁶ 145° C. XT⁶ 200° C. ST⁷80° C. XT⁷ 150° C. ST⁷ 110° C. XT⁷ 180° C. ST⁸ 50° C. XT⁸ 105° C. ST⁸ 80° C. XT⁸ 135° C. XT⁹ 105° C. XT⁹ 135° C. XT¹⁰  70° C. XT¹⁰ 100° C.XT¹¹  70° C. XT¹¹ 100° C. XT¹²  50° C. XT¹²  80° C.

It will be understood that the temperature ranges given herein for thebiomass slurry (ST) and heat exchange fluids (XT) at various points inthe system can vary in accordance with the particular process at hand.Appropriate temperatures for a given process will depend upon systemrequirements, pretreatment requirements, the particular lignocellulosicbiomass undergoing pretreatment, and other factors. It will also beunderstood that while those temperature ranges given above illustratecertain embodiments of the invention, other embodiments with othertemperature ranges are also encompassed by broader aspects of theinvention. As well, the number of heat exchangers and stages of heatingand cooling in the system can vary from those disclosed in system 30without departing from the spirit and scope of broader aspects of thepresent invention.

The aqueous lignocellulosic biomass can be passed through the heatexchanger(s) of the system at any suitable flow rate. Flow rates ofabout 5 gallons (US) per minute (gal/min) to about 200 gal/min will betypical, with flow rates of about 40 gal/min to about 100 gal/min beingmore preferred. In addition, in portions of the pretreatment system(e.g. in some or all heat exchangers) in which the biomass slurry isprocessed at pressures above the saturation vapor pressure of water orother liquid, the pressure will typically be in the range of about 50 toabout 500 pounds per square inch (psi).

The aqueous lignocellulosic biomass is pretreated by a heating andcooling cycle, but it is desirable to control the heat history of thelignocellulosic biomass in these systems. Overly extended heating cancause degradation of the lignocellulosic biomass, which can result inchemical degradation products that can inhibit the enzymatic processesused for the production of useful products from the lignocellulosicbiomass. In accordance with embodiments of the invention, thelignocellulosic biomass can be passed through the shell portion of theheat exchanger to provide relatively rapid and effective heating andcooling rates to achieve pretreatment of the lignocellulosic biomass,without causing significant undesired chemical degradation of thelignocellulosic biomass, even utilizing relatively high shell volume(biomass side) to core volume (heat transfer fluid side) ratios, forexample such ratios of at least about 3:1, or at least about 5:1,typically in the range of about 5:1 to about 50:1, and more preferablyabout 10:1 to about 30:1.

Other preferred aspects of the system 30 are illustrated in FIG. 2. Forexample, countercurrent heat recovery can be incorporated into thesystem. In particular, a single heat exchange fluid can be circulatedthrough heat exchangers 35 and 36 during which the heat exchange fluidpicks up heat from the aqueous biomass slurry during its cooling phase,and that same fluid can then be circulated through heat exchanger 31 totransfer heat to the aqueous slurry during its heating phase. Also, hotwater from liquid water source 43 can be circulated through heatexchanger 32 during which it transfers heat to the aqueous biomassslurry during its heating phase, and then through heat exchanger 34during which the liquid water picks up heat from the aqueous slurryduring its cooling phase. These and other arrangements of the heatingand cooling stages of the system 30 will be apparent to those skilled inthe art from the descriptions herein, and can be used in embodiments ofthe invention. Further, it will be understood that pretreatmentprocesses as described herein can be used in combination with otherpretreatment processes designed to increase the susceptibility of thebiomass to enzymatic hydrolysis, including for example chemical,mechanical and/or microbiological pretreatment processes.

In additional embodiments of the invention, a heat exchanger useful forpretreating a biomass slurry defines at least one helical groove alongat least a portion of its length, and potentially the entire length ofan outer flow passage of the heat exchanger. With reference to FIG. 3,shown is a perspective view of a tube-in-tube arrangement of oneembodiment of such a heat exchanger. The arrangement for heat exchanger50 thus includes at least one outer passage 51, at least one innerpassage 52, and a helical groove 53 defined along an outer wall 54 ofouter passage 51. Helical groove 53 can, for example, provide asecondary, helical passage 55 having a radial dimension R¹ that is atleast about 10% of that of the radial dimension R² of the primary outerpassage 56, with radial dimension R¹ typically being in the range ofabout 10% to about 50% of that of radial dimension R², more typically inthe range of about 10% to about 20%. Helical groove 53 can serve toenhance mixing of the aqueous biomass slurry as it passes through theoverall outer passage 51 including primary passage 56 and helicalpassage 55, so as to help to prevent plugging of outer passage 51. Itwill be understood that helical groove 53 can be used instead of or inaddition to fins, baffles, or other turbulence-inducing members locatedin outer passage 51. Heat exchanger(s) 50 can be used in overall systemscontaining a plurality of heat exchangers, e.g. a system 30 as disclosedin FIG. 2, and in so doing can be used for one, two or more, or all heatexchangers of the system.

In accordance with other aspects of the invention, lignocellulosicbiomass that has been pretreated as described herein can be utilized toproduce useful products, such as ethanol. In the production of ethanol,the pretreated biomass can be subjected to enzymatic hydrolysis with acellulase enzyme. In this regard, a cellulase enzyme is an enzyme thatcatalyzes the hydrolysis of cellulose to products such as glucose,cellobiose, and/or other cellooligosaccharides. Cellulase enzymes may beprovided as a multienzyme mixture comprising exo-cellobiohydrolases(CBH), endoglucanases (EG) and beta-glucosidases (betaG) that can beproduced by a number of plants and microorganisms. The process of thepresent invention can be carried out with any type of cellulase enzymes,regardless of their source; however, microbial cellulases providepreferred embodiments. Cellulase enzymes can, for example, be obtainedfrom fungi of the genera Aspergillus, Humicola, and Trichoderma, andfrom the bacteria of the genera Bacillus and Thermobifida.

Following enzymatic hydrolysis of the pretreated biomass, an aqueousmedium containing the resulting sugars can be subjected to fermentationto produce ethanol. In certain modes of practice, unhydrolyzed solids,typically including lignin, can be separated from liquids as abyproduct, for example by centrifugation or in a settling tank. Theunhydrolyzed solids can be sold into commercial channels such as feedindustries or combusted to generate thermal energy to be provided tosystems and methods of the invention.

The fermentation of the sugars to produce ethanol can be conducted withany of a wide variety of fermentive microorganisms such as yeast orbacteria, including genetically modified versions thereof, and usingknown techniques. The ethanol can then be purified from the fermentedmedium, for example by distillation.

EXAMPLE 1

Heat up profiles were obtained for aqueous lignocellulosic biomass in atube and shell heat exchanger. The aqueous lignocellulosic biomass wasprepared by mixing corn dry mill byproduct (lignocellulosic biomass)with water to make a slurry, which contained 22.8% solids (corn dry millbyproduct wt/total wt).

For determining heat up rates for the aqueous lignocellulosic biomass inthe shell portion of the heat exchanger, the aqueous lignocellulosicbiomass (7.67 gallons of the 22.8% solids slurry of corn dry millbyproduct in water) was placed in the shell portion of a heat exchanger(100% filled shell portion). The heat exchanger was made with a 4″ shellof schedule 40 pipe which was 155.75″ in length, with a 1″ schedule 80pipe running along the central axis to supply heat with a heat transferarea of 2190 sq. in. Steam (200 psig) was passed through the centraltube of the heat exchanger to heat the lignocellulosic biomass in theshell portion of the heat exchanger. The heat up profile is shown inFIG. 4, displaying a heat up time of 7 minutes for the lignocellulosicbiomass going from room temperature to 160° C.

For determining heat up rates for the aqueous lignocellulosic biomass inthe tube portion of a heat exchanger, the aqueous lignocellulosicbiomass (4.98 gallons of the 22.8% solids slurry of corn dry millbyproduct in water) was placed in the tube portion of a heat exchanger(100% filled). The heat exchanger was made with of an 8″ shell ofschedule 40 pipe which was 143.75″ in length, with a 3″ schedule 40 piperunning along the central axis to supply heat, with a heat transfer areaof 3000 sq. in. Steam (200 psig) was passed through the shell portion ofthe heat exchanger to heat the lignocellulosic biomass in the tubeportion of the heat exchanger. The heat up profile is shown in FIG. 5,displaying a heat up time of 6 minutes for the lignocellulosic biomassgoing from room temperature to 160° C.

The uses of the terms “a” and “an” and “the” and similar references inthe context of describing the invention (especially in the context ofthe following claims) are to be construed to cover both the singular andthe plural, unless otherwise indicated herein or clearly contradicted bycontext. Recitation of ranges of values herein are merely intended toserve as a shorthand method of referring individually to each separatevalue falling within the range, unless otherwise indicated herein, andeach separate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein, isintended merely to better illuminate the invention and does not pose alimitation on the scope of the invention unless otherwise claimed. Nolanguage in the specification should be construed as indicating anynon-claimed element as essential to the practice of the invention.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected. In addition, all references cited hereinare indicative of the level of skill in the art and are herebyincorporated by reference in their entirety.

What is claimed is:
 1. A method for processing lignocellulosic biomass,comprising: providing lignocellulosic biomass; providing a first heatexchanger including a first heat exchanger inner passage received withina first heat exchanger outer passage, with the first heat exchangerinner passage in heat exchange relationship with the first heatexchanger outer passage, and wherein the first heat exchanger outerpassage has a volume that is greater than that of the first heatexchanger inner passage, and wherein the first heat exchanger outerpassage has an inlet and an outlet; first passing an aqueous biomassslurry including said lignocellulosic biomass at a solids level of atleast 100 grams per liter through the first heat exchanger outer passagefrom the inlet of the first heat exchanger outer passage to the outletof the first heat exchanger outer passage while passing a heat exchangefluid through the first heat exchanger inner passage so as to heat theaqueous biomass slurry, wherein said first passing heats the aqueousbiomass slurry to a first temperature in the range of 70° C. to 120° C.;providing a second heat exchanger including a second heat exchangerinner passage received within a second heat exchanger outer passage,with the second heat exchanger inner passage in heat exchangerelationship with the second heat exchanger outer passage, wherein thesecond heat exchanger outer passage has a volume that is greater thanthat of the second heat exchanger inner passage, and wherein the secondheat exchanger outer passage has an inlet and an outlet, with the inletof the second heat exchanger outer passage connected in series to theoutlet of the first heat exchanger outer passage; after said firstpassing, second passing the aqueous biomass slurry through the secondheat exchanger outer passage from the inlet of the second heat exchangerouter passage to the outlet of said second heat exchanger outer passagewhile passing a heat exchange fluid through the second heat exchangerinner passage, wherein during said second passing the aqueous biomassslurry, the biomass slurry is at a second temperature exceeding 100° C.and under a pressure substantially equal to or exceeding the saturationvapor pressure of water at said temperature; during said second passing,exchanging heat between the aqueous biomass slurry in the second heatexchanger outer passage and the heat exchange fluid in the second heatexchanger inner passage to cause heating of the aqueous biomass slurryto the second temperature and for a duration within the second heatexchanger outer passage to increase susceptibility of thelignocellulosic biomass to hydrolysis by a cellulase enzyme; during saidsecond passing, feeding a liquid aqueous medium into the second heatexchanger outer passage through a plurality of openings atlongitudinally spaced positions between the inlet and the outlet of thesecond heat exchanger outer passage so as to locally regulate pressurewithin the second heat exchanger outer passage and reduce incidence oflocalized pressure drops and water hammer in the second heat exchangerthat would otherwise occur under conditions of said second passing step;and maintaining the aqueous biomass slurry under a pressuresubstantially equal to or exceeding the saturation vapor pressure of theaqueous biomass slurry during a pressurized flow of the aqueous biomassslurry from the inlet of the first heat exchanger outer passage to theoutlet of the second heat exchanger outer passage.
 2. The method ofclaim 1, wherein said aqueous biomass slurry includes thelignocellulosic biomass at a solids level of about 100 grams per literto 300 grams per liter.
 3. The method of claim 1, wherein the secondheat exchanger comprises a shell portion providing said second heatexchanger outer passage and a core portion providing said second heatexchanger inner passage.
 4. The method of claim 1, wherein said secondheat exchanger is a tube-in-tube heat exchanger, with a single innertube providing said second heat exchanger inner passage and a singleouter tube providing said second heat exchanger outer passage.
 5. Themethod of claim 3, wherein said second heat exchanger has a volume ratioof said shell portion to said core portion of at least about 3:1.
 6. Themethod of claim 5, wherein said second heat exchanger has a volume ratioof said shell portion to said core portion of about 10:1 to about 30:1.7. The method of claim 1, wherein said second heat exchanger outerpassage has a volume at least three times that of said inner passage. 8.The method of claim 1, wherein said second passing the aqueous biomassslurry is conducted at a flow rate of about 5 gallons (US)/min to about200 gallons(US)/min, and wherein said second heat exchanger has ahelical groove defined along an outer wall of the second heat exchangerouter passage and sized and configured to enhance mixing of the aqueousbiomass slurry as it passes through the second heat exchanger outerpassage.
 9. The method of claim 1, wherein said lignocellulosic biomasscomprises corn fiber or corn stover.
 10. The method of claim 1, whereinsaid second temperature exceeds about 150° C.
 11. A method forpretreating lignocellulosic biomass to increase its susceptibility toenzymatic hydrolysis, comprising: providing particulate lignocellulosicbiomass; mixing the particulate lignocellulosic biomass with an aqueousmedium to prepare an aqueous biomass slurry including thelignocellulosic biomass at a lignocellulosic biomass solids level of atleast 100 grams per liter; providing a plurality of heat exchangers eachincluding an inner passage received within an outer passage, with saidinner passage in heat exchange relationship with said outer passage, andwith the outer passage having a volume greater than that of the innerpassage; first passing the aqueous biomass slurry through the outerpassage of a first heat exchanger of said plurality of heat exchangerswhile passing a heat exchange fluid through the inner passage of thefirst heat exchanger so as to heat the aqueous biomass slurry, whereinsaid first passing heats the aqueous biomass slurry to a firsttemperature in the range of 70° C. to 120° C.; after said first passing,second passing the aqueous biomass slurry through the outer passage of asecond heat exchanger of said plurality of heat exchangers from an inletto an outlet of the outer passage of the second heat exchanger whilepassing a heat exchange fluid through the inner passage of the secondheat exchanger, wherein during said second passing the biomass slurry isheated to a second temperature greater than said first temperature,wherein the aqueous biomass slurry is maintained under a pressuresubstantially equal to or exceeding the saturation vapor pressure ofwater at said second temperature, and wherein during said second passingthe lignocellulosic biomass is modified to increase susceptibility tohydrolysis by a cellulase enzyme; during said second passing, feeding aliquid aqueous medium into the outer passage of the second heatexchanger through a plurality of openings at longitudinally spacedpositions between the inlet and the outlet of the outer passage of thesecond heat exchanger so as to locally regulate pressure within theouter passage of the second heat exchanger and reduce incidence oflocalized pressure drops and water hammer in the second heat exchangerthat would otherwise occur under conditions of said second passing step;and after said first passing and said second passing, third passing theaqueous biomass slurry through the outer passage of a third heatexchanger of said plurality of heat exchangers while passing a heatexchange fluid through the inner passage of the third heat exchanger,wherein during said third passing the aqueous biomass slurry is cooled;and maintaining the aqueous biomass slurry under a pressuresubstantially equal to or exceeding the saturation vapor pressure of theaqueous biomass slurry during a pressurized flow of the aqueous biomassslurry through a flow path including the outer passage of the first heatexchanger, the outer passage of the second heat exchanger and the outerpassage of the third heat exchanger.
 12. The method of claim 11, whereinsaid first passing and second passing the aqueous biomass slurry areconducted at a flow rate of about 5 gallons(US)/min to about 200gallons(US)/min.
 13. The method of claim 11, wherein at commencement ofsaid first passing, the aqueous biomass slurry has a temperature in therange of about 50° C. to about 90° C.
 14. The method of claim 11,wherein said second temperature exceeds about 150° C.
 15. The method ofclaim 11, wherein said aqueous biomass slurry includes thelignocellulosic biomass at a solids level in the range of about 100grams per liter to about 300 grams per liter.
 16. The method of claim11, wherein said heat exchanger has a volume ratio of said shell portionto said core portion of at least about 3:1.
 17. A method for processinglignocellulosic biomass, comprising: providing particulatelignocellulosic biomass; providing a first heat exchanger, a second heatexchanger and a third heat exchanger each including an inner passagereceived within an outer passage, with said inner passage in heatexchange relationship with said outer passage, and with the outerpassage having a volume that is greater than that of the inner passage;first passing an aqueous biomass slurry including said lignocellulosicbiomass at a lignocellulosic biomass solids level of at least 100 gramsper liter through said outer passage of said first heat exchanger whilepassing a heat exchange fluid through said inner passage of said firstheat exchanger so as to heat the aqueous biomass slurry, wherein saidfirst passing heats the aqueous biomass slurry to a first temperature inthe range of 70° C. to 120° C.; after said first passing, second passingthe aqueous biomass slurry through said outer passage of said secondheat exchanger from an inlet to an outlet of said outer passage of saidsecond heat exchanger while passing a heat exchange fluid through saidinner passage of said second heat exchanger so as to heat the aqueousbiomass slurry, wherein during said second passing the aqueous biomassslurry is heated to a second temperature exceeding the firsttemperature, and is maintained under a pressure substantially equal toor exceeding the saturation vapor pressure of water at said secondtemperature; during said second passing, feeding a liquid aqueous mediuminto said outer passage of said second heat exchanger through aplurality of openings at longitudinally spaced positions between saidinlet and said outlet of said outer passage of said second heatexchanger so as to locally regulate pressure within said outer passageof said second heat exchanger and reduce incidence of localized pressuredrops and water hammer in said second heat exchanger that wouldotherwise occur under conditions of said second passing step; after saidfirst passing and said second passing, third passing the aqueous biomassslurry through said outer passage of said third heat exchanger whilepassing a heat exchange fluid through said inner passage of said thirdheat exchanger, wherein during said third passing the aqueous biomassslurry is cooled; maintaining the aqueous biomass slurry under apressure substantially equal to or exceeding the saturation vaporpressure of the aqueous biomass slurry during a pressurized flow of theaqueous biomass slurry through a flow path including said outer passageof said first heat exchanger, said outer passage of said second heatexchanger and said outer passage of said third heat exchanger; andwherein during said first passing, said second passing, and said thirdpassing, said aqueous biomass slurry is maintained at a pH in the rangeof about 5 to about
 8. 18. The method of claim 17, wherein the firstheat exchanger is a core and shell heat exchanger with the core having aplurality of core passages providing said inner passage.
 19. The methodof claim 17, wherein the aqueous biomass slurry includes thelignocellulosic biomass at a solids level of 100 grams per liter toabout 300 grams per liter, and wherein said second temperature exceeds150° C.